Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 120

state. This is probably because the formation of polarons or polaron pairs is more
efficient than that of the singlet fission because of the larger interchain interaction
in highly ordered crystalline RR-P3HT films.
Finally, we note the relevance of singlet fission to polymer solar cells. As
mentioned above, singlet fission produces two triplet excitons from one singlet
exciton. This is analogous to multiple exciton generation in semiconductor
quantum dots [ 53 , 54 ], which has attracted increasing interest because more than
one excitons could be generated by one photon absorption. Recent studies have
demonstrated that the singlet fission indeed contributes to the photocurrent in
pentacene/C 60 bilayered films [ 55 , 56 ]. The singlet fission in RRa-P3HT films is
not directly linked with the polaron formation. This is partly because the triplet
exciton state is located lower in energy than the polaron state. Although further
studies are needed for efficient singlet fission into triplet excitons followed by
efficient charge generation, it is a challenging and attractive target to develop
polymer solar cells based on singlet fission.
5.6 Charge Dynamics
Let us move on to the dynamics of charge species generated in polymer-fullerene
blends. In particular, we focus on the charge dynamics in P3HT:PCBM blends and
summarize the relevance to the device performance [ 19 , 20 ].
5.6.1 Charge Generation
In RRa-P3HT:PCBM blend films, as shown in Fig. 5.6 a, P3HT singlet exciton is
already quenched to *50 % even at 0 ps, and disappears rapidly in a picosecond,
followed by P3HT polaron generation. Figure 5.13 a shows the time evolution of
P3HT singlet excitons and polarons in the RRa-P3HT:PCBM blend film. The
P3HT singlet exciton decays monoexponentially with a time constant of 0.2 ps. On
the other hand, more than half of the polarons are promptly generated even at 0 ps
and the remaining polarons are also rapidly generated with the same rise constant
of 0.2 ps. This agreement suggests that polarons are efficiently generated from
P3HT singlet excitons in a picosecond. The rapid formation of polarons is ascribed
to the prompt charge generation at the interface of RRa-P3HT/PCBM, because the
exciton migration is negligible on such a short time scale (\0.2 ps). This is
probably because PCBM molecules are more homogeneously dispersed in
RRa-P3HT amorphous films than in RR-P3HT crystalline films. Assuming a
homogeneous distribution of 50 wt % PCBM in RRa-P3HT films, the intermo-
lecular distance of neighboring PCBM molecules would be less than 1 nm. As
mentioned in Sect. 5.5.1 , the delocalization radius of singlet excitons is estimated
to be 3.2 nm in the RRa-P3HT film. This suggests that singlet excitons generated